1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to an apparatus for continuously supplying lithographic printing material to the plate cylinder of a planographic printing press or a plate imager.
2. Description of the Related Art
Traditional techniques of introducing a printed image onto a recording material include letterpress printing, gravure printing and offset lithography. All of these printing methods require a plate, usually loaded onto a plate cylinder of a rotary press for efficiency, to transfer ink in the pattern of the image. In letterpress printing, the image pattern is represented on the plate in the form of raised areas that accept ink and transfer it onto the recording medium by impression. Gravure printing plates, in contrast, contain series of wells or indentations that accept ink for deposit onto the recording medium; excess ink must be removed from the plate by a doctor blade or similar device prior to contact between the plate and the recording medium.
In the case of offset lithography, the image is present on a plate or mat as a pattern of ink-accepting (oleophilic) and ink-repellent (oleophobic) surface areas. In a dry printing system, the plate is simply inked and the image transferred onto a recording medium; the plate first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other copying medium. In typical rotary press systems, the recording medium is attached to an impression cylinder, which brings it into contact with the blanket cylinder.
In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening (or "fountain") solution to the plate prior to inking. The fountain solution prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
The plates for an offset printing press are produced photographically or through digital imaging. Traditionally, plates have been affixed to the plate cylinders of the press by means of clamps and the like. More recent systems, however, eliminate the chore of removing and replacing spent plates by locating a continuous supply of imageable plate material within a cavity within the plate cylinder. Each time a printing job is completed, fresh plate material is advanced around the cylinder to replace the spent segment.
It is important, during press operation, to maintain a substantial tension along the plate material that surrounds the plate cylinder. This material experiences significant tangential force as a result of contact with the blanket cylinder, the force resulting primarily from slight differences in the rolling diameters of the mating cylindrical surfaces, which are in contact at sufficient pressure to compress the compliant blanket cylinder surface, and will alter the orientation of the plate or dislodge it completely unless the plate is held with adequate tension against the plate cylinder. Accordingly, a plate-material "payout" system must maintain strong contact between the plate material and the cylinder; at the same time, however, it must also allow sufficient relaxation to permit smooth supply and uptake of the material. U.S. Pat. Nos. 5,355,795 and 5,727,749 (the entire disclosures of which are hereby incorporated by reference) teach arrangements designed to accommodate the tensioning requirements of commercial printing systems.
Such systems, however, provide for only a single segment of plate material that is imaged, used to print copy, and replaced with fresh material drawn from inside the plate cylinder. Although a single segment of plate material may accept multiple images (e.g., to facilitate simultaneous printing of different jobs, or multiple pages of the same job), all must be discarded at the same time as the material is advanced.